Someone on StackOverflow stumbled onto the strange term “open recursion” and asked what it meant. Since most of the other answers to this online are pretty opaque, I started writing an answer. But then I accidentally wrote a blog post.

I honestly couldn’t remember what it meant either, so I cracked open my copy of Types and Programming Languages where, I believe, Pierce first introduces the term. After skimming a bit, I think I’ve got it. For those who don’t have the book or don’t want to wade through PL nerd terminology, I’ll try to translate it to something a little friendlier.

First, a bit of context. Pierce is explaining the semantics and types of object-oriented languages starting from a non-OOP core based on the lambda calculus and records. He starts the book with the simplest possible proto-language and then keeps adding extensions to it to build up to the kind of languages we see today. He coined “open recursion” to refer to the kind of extensions you need to build an OOP language from a non-OOP one that just has functions (i.e. “lambdas”, “closures”, or “anonymous delegates”) and records (more or less “object literals” in JS or “maps” in other languages).

Since not too many people know the lambda calculus, for this scene I will use a subset of Dart as its stand-in. We’ll allow function declarations and maps, but no actual classes or methods.

Now the question is, if you were to just have maps of functions, what would you be missing compared to “real” objects? Pierce’s answer is “open recursion”. We’ll break that down into the two words, last one first:

“Recursion”

Say you want to make an “object” that represents a counter. It exposes three operations: increment, get, and set. You could make such an object in our Dart subset like this:

makeCounter(){// Declare the instance state for the "object".varcount=0;// Declare functions for the "methods". They are closures, so they can// access count.increment(){count++;}get(){returncount;}set(value){count=value;}// Make an "object" as a map of functions.return{'get':get,'set':set,'increment':increment};}

Great. This works fine. But let’s say we wanted to implement increment in terms of get and set. One common feature of methods is that they can call each other. Let’s try:

makeCounter(){// Declare the instance state for the "object".varcount=0;// Declare functions for the "methods". They are closures, so they can// access count.increment(){set(get()+1));}get(){returncount;}set(value){count=value;}// Make an "object" as a map of functions.return{'get':get,'set':set,'increment':increment};}

Oops! This doesn’t work. The problem is that increment is calling get and set here, but those functions haven’t been declared yet. Unlike JavaScript, Dart doesn’t silently hoist function declarations up. So at the point that we’re defining increment, get and set aren’t declared.

We could move increment after get and set to fix this issue. But then those two methods wouldn’t be able to see increment. No matter what, there’s no way to have all of those functions in scope inside each of the other ones.

The problem is that the definitions of the functions come one after the other. What you really want is to define them all in one lump where they can all see each other simultaneously.

In functional languages, the name for that “lump” is a “mutually recursive definition”. It lets you declare a bunch of variable names and refer to those names within the definitions of each of those variables. In Scheme and ML, this is the difference between let and letrec (the rec in the name stands for “recursive”). In C, you need forward declarations to do this.

So by “recursion” here, what he means is the definitions of the methods are mutually recursive so that they can see each other’s names. It doesn’t mean that they actually have to call each other at runtime and be recursive. Just that their names are in scope so that they could do that.

“Open”

We can fake mutually recursive definitions in our mini-Dart by using function expressions and reassigning variables like this:

makeCounter(){// Declare the instance state for the "object".varcount=0;// Declare the variables up front.varincrement,get,set.// Now that the names are all in scope, create the function bodies.increment=(){set(get()+1));};get=(){returncount;};set=(value){count=value;};// Make an "object" as a map of functions.return{'get':get,'set':set,'increment':increment};}

Note the = between the method names and () now. That means we’re assigning anonymous functions to the already-declared variables.

So this gives us recursive structures. Do we have objects yet? What’s missing?

Another defining feature of object-oriented languages is inheritance (or “delegation” in prototype-based languages). That means defining a new object in terms of an existing object’s behavior. That basically means overriding methods and adding new methods in “derived” objects.

Let’s try to do that. We’ll try to make a counter that logs itself. To avoid re-implementation, we’ll piggyback the existing counter code:

How did we do? When we call get and set on our logging counter, it does correctly print “get!” and “set!” and then update the counter appropriately. The problem comes when we call increment. That does print “increment!”. But, remember, increment is implemented in terms of get and set now.

Since we intended to override those methods in our logging object, calling incrementshould print “get!” and “set!” too. It doesn’t. That’s because the non-logging object’s implementation of increment is statically bound to the base definitions of those methods. We haven’t overridden them in our derived logging counter, we’ve just shadowed them.

In C++ parlance, they are non-virtual. Our mini-Dart language isn’t expressive enough to handle virtual methods. The problem is that inside makeCounter, we don’t see the instance of the logging counter at all. To fix this, we have to pass that object in explicitly:

makeCounter(receiver){// Declare the instance state for the "object".varcount=0;// Declare the variables up front.varincrement,get,set;// Now that the names are all in scope, create the function bodies.increment=(){receiver['set'](receiver['get']()+1));};get=(){returncount;};set=(value){count=value;};// Add the methods to the receiver.receiver['get']=get;receiver['set']=set;receiver['increment']=increment;}

Note how now increment’s definition looks up get and set on that passed in receiver object. Now to create the logging counter, we’ll do:

makeLoggingCounter(){// Create a blank object.counter={};// Turn it into a counter.makeCounter(counter);// Keep track of the original methods.varsuperGet=counter['get'];varsuperSet=counter['set'];varsuperIncrement=counter['increment'];// Override the methods.counter['get']=(){print('get!');returnsuperGet();};counter['set']=(value){print('set!');superSet(value);};counter['increment']=(){print('increment!');superIncrement();};returncounter;}

Ta-da!

Now, to make this work, we had to pass the receiver into makeCounter so that it’s methods could see the “derived” object. This lets it see and call overridden methods. Those methods are now effectively “virtual”.

Before we did this, the methods in makeCounterclosed over each other. In other words, they were all closures and called each other by closing over each other’s variables. By passing in the receiver explicitly, we’ve cracked open that closure and let the derived object get in so the base methods can see it. Hence: “open”.

Super summary

So, if you compare a real object-oriented language to a simpler language with just structures and functions, the differences are:

All of the methods can see and call each other. The order they are defined doesn’t matter since their definitions are “simultaneous” or mutually recursive.

The base methods have access to the derived receiver object (i.e. this or self in other languages) so they don’t close over just each other. They are open to overridden methods.